Dry friction clutch/brakes depend upon the rubbing of a dry friction material against dry reaction members to start and stop a machine tool. This continuous dry rubbing causes wearing of both the friction material and the reaction members as well as causing the generation of heat in these members. The faster the machine tool operates and the faster the flywheel rotates, greater is the amount of wear and heat which are generated. This generation of wear and heat requires periodic gap adjustment between the friction plates and the reaction members to keep the clutch/brakes functioning and the machine tool operating correctly. The trip rate or cycle speed of a machine tool equipped with a dry friction clutch/brake unit is limited because the mass of the unit determines its heat dissipation capacity. If the mass of the unit is increased to increase its heat dissipation capacity, the inertia that must be started and stopped is also increased. These factors define a closed loop from which it is impossible to escape when trying to substantially increase the performance of the system.
Oil shear brake and clutch units have been developed to eliminate the problems associated with the dry friction type of units. The oil shear systems use hydraulic actuation instead of air actuation and the units have been developed with key modules which can be easily adapted to be mated with the various types of machine tools. Properly designed oil shear clutch/brake drives offer the advantage of little or no wear of plates in the disk stacks and no brake fade. This provides a more precise operation of the machine tool and dramatically increases machine tool up-time. The oil film between the adjacent disks carries the heat generated by the start-stops of the unit away from the disk stacks. This removal of heat offers the advantage that there is now no practical limit for the machine tool trip rate and the flywheel speed. In addition, this removal of heat provides unlimited inching capabilities.
While the oil shear clutch/brake drive units have significantly improved the operation of machine tool drives, they present totally new issues which must be addressed. One of these issues is providing an adequate supply of oil to the clutch and brake assemblies of the drive. Prior art lubrication systems supply oil to the inside portion of the drive (near the axis of rotation) and subsequently remove the oil from the outside portion of the drive. The rotation of the components of the drive and the associated centrifugal force acting on the oil in these prior art drives is thus in the direction of the oil flow. There is therefore, a tendency in the prior art to increase the flow rate of the oil. As long as a sufficient supply of lubricant is continuously supplied to the drive, these prior art drives perform satisfactorily. Should there be a momentary delay in the supply of oil to the inside portion of the drive, the friction plates which are located on the outside portion of the drive would be momentarily starved of oil. This starving of oil will result in the generation of heat and excessive wear of the friction plates.
Various designs have been developed in an attempt to eliminate this type of a problem. Prior art drives have been developed which reverse the direction of low of the oil. These prior art drives supply the oil to the outside portion of the drive and remove the oil near the axis of rotation or inside portion of the drive. The centrifugal force acting on the oil due to the rotation of various components of the drive is thus against the flow of the oil and these drives have proven to be more accommodating to a momentary delay in the supply of oil.
While reversing the flow of oil has significantly improved the performance of machine tool drives incorporating oil shear clutches and brakes, the continued development of lubrication systems for ensuring a continuous and adequate supply of lubricant to the friction plates of the clutch and brake assemblies.